It is well known to provide video security systems in which video cameras are used to generate video signals representative of locations for which security surveillance is desired. In a typical system, some or all of the video signals are displayed on video screens for monitoring by security personnel. It is also known to record some or all of the video signals on videotape, either to provide evidentiary support for the observations of security personnel or in cases where "real-time" human monitoring of the signals is impractical or is not desired.
However, videotape suffers from serious drawbacks as a storage medium, particularly in view of the large quantity of video information generated by video security systems. A major concern is the sheer quantity of tapes to be stored, especially when it is desired to record signals generated by a large number of surveillance cameras. Moreover, in a large system many videotape recorders may be required, resulting in a large capital expenditure, and also the need for allocate space for the recorders. Another problem is the need to frequently change tape cassettes.
Retrieving information of interest from recorded tapes presents additional challenges. It is the nature of video surveillance that a large part of the tape-recorded video surveillance signal is of no interest whatsoever, since it typically represents a static image of a hall-way or the like. Finding a particular sequence representing a significant event can be extremely difficult and time-consuming, requiring tedious human review of hours or days of tape-recorded signals.
There have been a number of attempts to overcome these disadvantages, but so far with limited success, or at the cost of additional drawbacks. For example, it is known to use "quad multiplexers" to combine signals from four video cameras into a single dynamic image, having four quadrants each dedicated to a respective one of the cameras. The resultant space-multiplexed signal can then be recorded, realizing a four-to-one compression ratio in terms of required storage capacity. However, the multiplexed image suffers from a corresponding loss of spatial resolution, which may impair the value of the recorded images as evidence or may interfere with subsequent review. Also, recording of multiplexed images does not address the problems involved in finding sequences of interest on the recorded tapes.
It is also known to record the surveillance video signals selectively in response to input from a human operator who is monitoring the signals or in response to signals generated by sensor devices arranged to detect events such as opening of doors or windows. This technique reduces the total information to be recorded, while preventing storage of much uninteresting information, but at the risk of failing to record significant events which cannot readily or timely be detected by sensors or human operators. Also, reliance on external input can result in unreliability and increased expense, particularly where human operators are to initiate recording.
The OPTIMA II video surveillance multiplexer introduced by the assignee of the present application employs a more sophisticated technique for culling out uninteresting information prior to storage. In the OPTIMA II multiplexer respective streams of video image information are received from a plurality of cameras and a combined stream of images is formed by time-division multiplexing of the images from the cameras. The combined stream is then output to a conventional videotape recorder for storage on tape. The OPTIMA II multiplexer applies motion detection analysis to the respective input streams and adaptively allocates the "time slots" in the output stream by allocating a larger number of slots to images from an input stream in which motion is detected. In this way, a relatively large portion of the system's storage capacity is allocated to image streams which contain moving objects and are therefore more likely to include useful information.
The OPTIMA II multiplexer represents a significant advance over conventional tape-based surveillance video storage techniques, but still greater efficiency and flexibility are to be desired.
The "MultiScop" video disc recorder sold by Geutebruck GmbH is an application of digital recording to the problem of storing video surveillance information. The MultiScop system employs the above-mentioned selective recording technique to minimize recording of "uninteresting" information. In addition, some redundant information is excluded from recording by use of a conventional digital image compression technique. Random access to stored information, based on date and time indexing, or based on indexing indicative of an externally sensed alarm condition, provides a modest improvement over conventional tape-based systems in terms of convenience in retrieving stored video.
However, greater efficiency and flexibility than are provided by the MultiScop system are to be desired. In particular, it would be most useful to exclude uninteresting information from recording while minimizing the chance of missing significant information. Also more efficient information retrieval techniques are needed.